1. Field of the Invention
The present invention relates to a toner and a developer composition used for a dry-type, two-component development method wherein electrostatic latent images are developed using image-forming apparatuses such as laser printers and dry-type electrostatic copying machines. More particularly, it relates to a toner and a developer composition capable of maintaining carrier resistance and triboelectric charge with substantially no change for a long period of time by reducing the toner scum of the carrier, thereby maintaining high image quality.
2. Discussion of the Related Art
As described in U.S. Pat. Nos. 2,221,776, 2,297,691 and 2,357,809 and other publications, the conventional electrophotography using an image-forming apparatus such as a dry-type electrostatic copying machine comprises the steps of evenly charging a photoconductive insulating layer (a charging process); subsequently exposing the layer to eliminate the charge on the exposed portion so as to form an electrostatic latent image (an exposing process); visualizing the formed image by adhering colored charged fine powder known as a toner to the latent image (a developing process); transferring the obtained visible image to an image-receiving sheet such as a transfer paper (a transfer process); and permanently fixing the transferred image by heating, pressure application or other appropriate means of fixing (a fixing process).
As stated above, a toner must meet the requirements not only in the development process but also in the transfer process and fixing process.
As suitable development methods for electrophotography, there are basically two kinds, i.e., a dry-type development method and a wet-type development method. In the dry-type development method, there are a method utilizing a one-component system developer and a method utilizing a two-component system developer comprising a toner and a carrier.
As methods utilizing two-component system developers, a magnetic brush development utilizing a magnetic powder carrier (see U.S. Pat. No. 2,786,439) and a cascade development utilizing a bead carrier with a relatively large particle size (see U.S. Pat. No. 2,618,551) have been known, the above method having two different conveying systems of toners.
As toners used for such development methods, it has generally known to use fine particles obtained by the steps of mixing and dispersing various functional additives including coloring agents, such as dyes and pigments, charge control agents and waxes in binder resins such as styrene-acrylate copolymer (polystyrene type) resins, polyester resins and epoxy resins, and then pulverizing the obtained dispersion mixture to a particle size of 1 to 80 .mu.m.
For the two-component system developers mentioned above, long service life, specifically, stability and retainability of image quality in continuous development, is generally in demand. In order to achieve the long service life required for the developer, triboelectric charge and developer resistance for the toner and the carrier used have to be maintained within suitable ranges. It is particularly desired that the initial values of the triboelectric charge and developer resistance do not change even at the end of the long-time durability test.
For the purpose of stably maintaining the triboelectric charge and developer resistance of these developers, various investigations have been made from the toner side as to the designs and choices of binder resins, charge control agents and other additive, and also from the carrier side as to the oxidation treatment methods for iron particles, essential materials constituting carriers such as ferrites and magnetites, surface shapes of these magnetic particles, coating materials and treatment methods therefor.
Particularly in the case where a high-speed development is carried out, since a large amount of developer has to be used, a large mechanical force for stirring the developer is required, so that the developer is drastically deteriorated by the repetition of collision of the developer and the stirring member of the developer device.
In order to reduce such a deterioration of the developer, the bulk density of the carrier can be made small by using porous, irregular-shaped iron particles which is a so-called "sponge" as a carrier, so that the stirring torque can be made lower. As the developing speed increases, carriers existing in the vicinity of the photoconductor are likely to be "charged up" at development, thereby making it likely to increase the edge effects of the solid image portion. This problem can be solved by such subsidiary effects as increase in the electric field strength in development. The reasons for the increase in the electric field strength are as follows: The presence of the porous, irregular-shaped iron particles provides an increase in contact points between the particles, so that the bias current is increased, which in turn increases the electric field strength. Therefore, the carriers having such a structure are highly suitable for high-speed printers and copying machines utilizing semiconductive methods.
Further, when a carrier is made porous, the low-surface energy coating material formed on the projecting portion of the carrier particles is detached during repetitive use thereof, which in turn leads to generate toner fine particles in the developer device and to constitute the nucleus, so that a thin layer which is a so-called "toner scum" is formed onto the bare surface, thereby increasing the surface frictional resistance of the carrier. Therefore, it would be difficult to make development bias voltage reach the tip end of brush near the photoconductor surface, thereby decreasing the image density of the formed image, particularly making it difficult to reproduce the solid image portion. Also, particularly in the case of using high-speed copying machines and printers, low-temperature fixing ability of the toner is required, but in the case where the temperature in the developer device reaches not less than 50.degree. C. by its continuous use, the so called "toner scum phenomenon" is likely to take place, wherein the low-temperature fixing toner is thermally fused and solidified on the projecting portion of the carrier. Therefore, a low-temperature fixing ability and a long service life required for the developer cannot be simultaneously met satisfactorily.
FIGS. 1(a)-(c) are schematic views each showing the mechanism of the toner scum formation. In the figure, the carrier comprises a core 1 made of porous iron particles having a rugged surface, and a conductive coating material 2 formed on the surface of the core 1.
Specifically, FIG. 1(a) schematically shows a carrier in its initial state where dents of the core 1 are filled with the toner 3, but no toners are present on the projecting portions, so that these projecting portions effectively serve as contact points for adjacent carriers. However, as shown in FIG. 1(b), the coating material 2 present in the projecting portions is detached by frictional forces caused during the repetitive use of the carrier, so that toner scum is likely to take place, thereby forming toner scum 3a with toners 3 on these bare projecting portions as shown in FIG. 1(c).
It is well known that each of the carriers are linearly connected between the developing sleeve and the photoconductor by making the projecting portions mentioned above as the contact points, the carriers serving to supply charged toner 3 to an electrostatic latent image formed on the surface of the photoconductor. However, when the surface frictional resistance of the carrier is undesirably increased by the formation of the toner scum 3a, bias voltage applied is not easily conducted to the photoconductor through the developing sleeve, thereby resulting in the decrease in the amount of toners 3 supplied onto the surface of photoconductor, which in turn causes a decrease in the image density as described above.
As materials not only serving to improve cleaning ability of the toners using the carrier having this type of structure but also serving to prevent offset phenomenon and reduce friction of the photoconductor, it has been conventionally known to externally add a suitable amount of a lubricant such as zinc stearate to toner particles. For instance, as disclosed in Japanese Patent Laid-Open No. 46-12680, zinc stearate is externally added to a toner using a linear polyester as a binder resin in order to lubricate the outer surface of toner particles. In this method, even though the coating material on the projecting portion may wear out, since the toner particles are not easily adhered to the carrier surface, the decrease in image density after a long period of use can be prevented, thereby showing a long service life.
However, in the toner thus obtained by externally adding a lubricant, the lubricant is likely to be separated from the toner. Particularly in the transfer process, the lubricant which becomes detached from the toner is in turn adhered onto the surface of the photoconductor, thereby forming an insulating layer on the surface of the photoconductor during the continuous development. Moreover, the detachment of the lubricant gives drastic influence on the chargeability of the toner itself, which in turn generates image deterioration such as background due to the decrease in the triboelectric charge and the decrease of the developer resistance caused by improvement in fluidity of the developer.
Accordingly, in the two-component system developer using a carrier made of a porous, irregular-shaped iron powder having a rugged surface, a developer composition capable of stably maintaining carrier resistance and triboelectric charge for a long period of time while retaining good low-temperature fixing ability of the toner is in demand.